ALOHA
The Aloha protocol was designed as part of a project at the University of Hawaii. It provided data transmission between computers on several of the Hawaiian Islands using packet radio networks. Aloha is a multiple access protocol at the data link layer and proposes how multiple terminals can access the medium without interference or collision.
There are two different versions of Aloha:
1. Pure Aloha
Pure Aloha is an un-slotted, decentralized, and simple-to-implement protocol. In pure Aloha, the stations simply transmit frames whenever they want data to send. It does not check whether the channel is busy or not before transmitting. In the event that two or more stations transmit simultaneously, a collision occurs and frames are destroyed. Whenever any station transmits a frame, it expects acknowledgment from the receiver. If it is not received within a specified time, the station assumes that the frame or acknowledgment has been destroyed. Then, the station waits for a random amount of time and sends the frame again. This randomness helps in avoiding more collisions. This scheme works well in small networks where the load is not much. But in largely loaded networks, this scheme fails poorly. This led to the development of Slotted Aloha.
To assure pure aloha: Its throughput and rate of transmission of the frame are to be predicted.
For that, let’s make some assumptions:
- All the frames should be the same length.
- Stations can not generate frames while transmitting or trying to transmit frames.
- The population of stations attempts to transmit (both new frames and old frames that collided) according to a Poisson distribution.
Vulnerable Time = 2 * Tt
The efficiency of Pure ALOHA:
Pure Aloha= G * e^-2G
where G is number of stations wants to transmit in Tt slot.
Maximum Efficiency:
Maximum Efficiency will be obtained when G=1/2
(pure aloha)max = 1/2 * e^-1
= 0.184
Which means, in Pure ALOHA, only about 18.4% of the time is used for successful transmissions.
2. Slotted Aloha
This is quite similar to Pure Aloha, differing only in the way transmissions take place. Instead of transmitting right at the demand time, the sender waits for some time. In slotted ALOHA, the time of the shared channel is divided into discrete intervals called Slots. The stations are eligible to send a frame only at the beginning of the slot and only one frame per slot is sent. If any station is not able to place the frame onto the channel at the beginning of the slot, it has to wait until the beginning of the next time slot. There is still a possibility of collision if two stations try to send at the beginning of the same time slot. But still, the number of collisions that can possibly take place is reduced by a large margin and the performance becomes much well compared to Pure Aloha.
Collision is possible only in the current slot. Therefore, Vulnerable Time is Tt.
The efficiency of Slotted Aloha
slotted Aloha= G * e^-G
Maximum Efficiency:
(slotted)max = 1 * e^-1
= 1/e = 0.368
Maximum Efficiency, in Slotted ALOHA, is 36.8%.
What is Ethernet?
A local Area Network (LAN) is a data communication network connecting various terminals or computers within a building or limited geographical area. The connection between the devices could be wired or wireless. Ethernet, Token rings, and Wireless LAN using IEEE 802.11 are examples of standard LAN technologies.